1. Field of the Invention
This invention relates generally to methods for reducing NOx emissions from a combustion engine and apparatuses thereof.
2. Background Art
NOx (also commonly written as NOx or NOX) is the generic name for a group of highly reactive gases that contain varying amounts of nitrogen oxides including, but not limited to, NO, NO2, NO3, N2O, N2O3, N2O4, N3O4, and mixtures thereof. They are typically colorless and odorless but react readily with ammonia, moisture and other compounds to form nitric acid and related particles. NOx and their related chemical products have been identified as major air pollutants and are the causes for serious health and environmental concerns in the U.S. and other industrialized nations around the world. Some of the heath and environmental problems related to NOx include ground level ozone (smog) formed when NOx and volatile organic compounds (VOC) react in heat and sunlight, acid rains formed when NOx and sulfur oxide react with other substances in the air to form acids, and deep-penetrating particles that can exacerbate human respiratory diseases such as emphysema and bronchitis. According to some estimates, common sources of NOx are motor vehicles (49%), electric utilities (27%), and other industrial, commercial and residential sources (19%) that burn hydrocarbon fuels.
Among the common sources of NOx, motor vehicles have perhaps received the most attention. Since 1970, the U.S. Environmental Protection Agency (EPA) has required motor vehicle manufacturers to reduce NOx emissions. Although significant reductions have been achieved through auto emissions controls, increasing public awareness of environmental issues has led to ever more stringent regulatory requirements. On Dec. 21, 2000, the EPA signed emissions standards for model year 2007 and later heavy-duty highway engines to limit the emission level to about 0.20 g/bhp-hr, a ten-fold reduction from the 2004 level. In light of the great public interest in reducing NOx emissions, there is an urgent need for improved means to reduce NOx emissions. Therefore, there is a need for a device, system or method for reducing NOx emissions in combustion engines and for reducing NOx emissions from other NOx producing devices as well.
To address the problem of NOx emissions, an understanding of how NOx is formed may be helpful. During combustion of fossil fuels (mainly hydrocarbons) NOx is formed via several mechanisms: thermal NOx, fuel NOx, and prompt NOx. Thermal NOx results from the reaction between nitrogen in the air and excess oxygen at elevated temperatures. Fuel NOx results when nitrogen is oxidized by combustion air. Prompt NOx is caused by the intermediate formation of hydrogen cyanide (HCN) followed by the oxidation of HCN to nitric oxide (NO).
Fuel NOx and prompt NOx are typically not of major concern. Fuel NOx is not produced in significant amounts from burning of hydrocarbon fuels, which have little or no chemically bound nitrogen. Moreover, the nitrogen content of fuels can be reduced during the production of the fuel using known techniques. Prompt NOx is only significant in very fuel-rich flames and is produced by high-speed reactions in the flame front. Fuel rich flames are not prevalent in internal combustion engines. These two types of NOx will herein be collectively referred to as chemical NOx.
The formation of thermal NOx from a typical combustion reaction converting fossil fuel into energy is a complex chemical phenomenon involving about 1000 chemical reactions with about 100 distinct chemical species. Understanding this complex network of interacting reactions can be a daunting task and detailed theoretical modeling and prediction of combustion remain difficult (for an overview, see Chemical Kinetics and Combustion Modeling. J A Miller, R J Kee, C K Westbrook Annual Review of Physical Chemistry, October 1990, Vol. 41, Pages 345-387). However, some of the essential mechanisms of thermal NOx formation (this is sometimes referred to as the Zel'dovich mechanism) may be generally understood. A generalized description of the process is set forth below.
At high temperatures, both nitrogen (N2) and oxygen (O2) are dissociated into atoms that react by the Zel'dovich mechanism:N2+ONO+NN+O2NO+ON+OHNO+H
In this process, NO is the principal reaction product. Once nitrogen and oxygen molecules are dissociated into atoms, the indicated three reactions form a chain reaction process that can self-propagate for many cycles, each adding to the amount of NO produced. The major factors that affect thermal NOx production are combustion temperature, residence time at temperature, the degree of fuel/air mixing, and the concentrations of oxygen and nitrogen in the air that is burned. Higher temperature, longer residence time, enhanced mixing, and higher oxygen concentration all favor NOx formation.
As mentioned earlier, in addition to transportation related pollution, it is known that industrial, commercial, and residential burners such as those used in electric utilities, in commercial factories, and in residential home heaters can also contribute to the increasing global concentration of NOx. For purposes of this discussion, we will limit our focus primarily to NOx emissions resulting from transportation industries and particularly motor vehicle combustion engines including, but not limited to, gasoline and diesel engines.
In FIG. 1, a schematic diagram of a typical combustion engine 10 is shown. The engine 10 receives oxidant 12 for the combustion reaction (usually atmospheric air) and fuel 14 through the reactant intake 16. The fuel and oxidant may be fed individually for mixing in the cylinder or may mix in the reactant intake 16. Once the reactants are inside the combustion chamber 18 of the engine 10, the reactants (air-fuel mixture 20) are ignited, as for example, with a spark plug 22 during the compression stroke of the engine 10. The combustion results in the release of heat energy and expansion of gases that increases the pressure inside the combustion chamber 18. After the combustion reaction, the products of the reaction 24 are released from the chamber 18 through engine exhaust passage 26, typically into the atmosphere 30. The reaction products 24, sometimes referred to as exhaust gases, may contain unburned or partially burned hydrocarbons and also NOx, both of which contribute to pollution when they are exhausted into the atmosphere 30. In some cases, the exhaust passage 26 may comprise one or more devices to reduce the NOx that is discharged into the atmosphere. For example, there may be a scrubber 32 and/or a catalytic converter 34 included along or at the outlet of the exhaust passage 18. Devices such as scrubber 32 and catalytic converter 34 are designed to extract NOx from the exhaust gas stream 20 after the NOx has already been formed. Such devices can be complicated, difficult to install on internal combustion engines, and are often expensive. Typically, such devices are only partially effective to extract the NOx. Unburned hydrocarbon may also be found in the exhaust stream and different means are required for removing hydrocarbon compared to the means required for removing NOx.
Prior to the present invention, much of the research on reduction of emissions from combustion engines has been primarily focused on either completing combustion of all components of the fuel sources or on implementing post-combustion clean-up technologies. Complete combustion typically focuses on burning the fuel at high temperatures and burning the fuel with high oxidant content so that all the components of the hydrocarbon fuels are fully oxidized. The use of excess oxidants to ensure complete combustion is sometimes referred to as lean fuel/air ratio burning (less fuel than stoichiometricly balanced fuel combustion would require for the available oxygen in the air). The same situation is sometimes referred to as rich air/fuel combustion (more air than is required to combine completely with the available fuel). Both expressions represent the same situation where the amount of oxidants required for complete combustion is greater that the amount of fuel present. This is sometimes expressed with an inequality expression as “oxidant>fuel”. Some combustion modification technologies have been aimed at modifying the complex combustion chemistry such as by injecting additives into the fuel-mix to lower the production of NOx while maintaining complete combustion. One approach to reducing pollution has been to provide an enriched oxygen air stream into an internal combustion engine to accomplish complete combustion. While increased oxygen content in the air fuel mixture can decrease the unburned hydrocarbon pollution in the exhaust, it has been found that the increased oxidants often also increases the production of NOx.
It has also been common to consider post-combustion clean-up technologies that are aimed at extracting NOx from the engine exhaust using devices such as scrubbers and catalytic converters. Maintaining both complete combustion of the fuel and reduced NOx are often competing goals. In prior devices, complete hydrocarbon combustion and engine efficiency often meant that the NOx formation was increased and scrubbers and/or catalytic converters were relied upon to reduce NOx emissions. Moreover, when using scrubbers and catalytic converters it has often been necessary to periodically replace the expensive scrubbers and converters to ensure NOx extraction efficiency and to provide different sizes, numbers, or types of scrubbers or converters to upgrade the system to keep-up with the latest government mandated low emission requirements.